The present invention relates generally to hydraulic drive systems and more specifically to an add-on auxiliary drive system utilized on the steerable non-driven wheels of an agricultural tractor. Most farm tractors built today are of the conventional two-wheel drive type having two large mechanically driven rear wheels and a pair of smaller steerable front wheels which are not driven. Under most conditions, the conventional main drive wheels provide sufficient tractive effort to propel the tractor and pull the particular tillage implement. However, under certain conditions, additional tractive effort at varying levels is desirable when the main drive wheels begin to spin. Under such conditions quite often the conventional drive train is not utilizing the full horse power capacity of the tractor and that additional power capacity can be converted into hydraulic energy to drive the front steerable wheels of the tractor. This additional tractive force substantially boosts the drawbar pulling power of the tractor and also improves steering control. In varying soil conditions, it is also advantageous to be able to vary the amount of torque output from the auxiliary system.